1. Field of the Invention
This invention relates to gas turbine engines and, more particularly, to clearance control between rotating machinery and an outer shroud or seal.
2. Summary of the Prior Art
In the quest for improving the performance of current generation gas turbine engines, a great deal of development has been directed at harnessing the full potential of combustion gases by reducing the leakage of hot gas turbine air. One of these leakage areas is between tips of rotating turbine blades and an outer shroud or seal of the turbine compartment causing a substantial loss of turbine efficiency and an increase in fuel consumption. This leakage can be reduced if the engine is provided with a controlled means of cooling the turbine case which encloses and supports the outer shroud or seal. The cooling of this case by either internal or external means causes a thermal contraction around the tips of the rotating turbine blades and reduces blade tip clearance. This procedure is commonly known to those skilled in the art as clearance control and is currently being introduced into advanced aircraft engines to improve engine performance.
While the clearance control system for directing cooling airflow to the turbine case improves gas turbine performance, even greater benefits can be obtained by varying or modulating this flow of cooling air, thereby varying the clearance control. Because the engine operates at different speeds and temperatures, the clearance between turbine blades and the outer shroud or seal on engines without clearance control varies with the rotational speed of the blades and the gas temperatures within the engine. To maximize the performance of the engine, it is desirable to maintain the clearance between the blades and the shroud at a minimum value during various engine operations. By modulating the amount of cooling air directed onto the engine case, and matching the amount of cooling air with the present engine operation, lesser clearances can be obtained.
A further improvement in engine operation can be achieved by selective allocation of cooling airflow between the clearance control and other equipment or airspace within the engine system that can benefit from cooler temperatures.